Filename: 184-v3-link-protocol.txt
Title: Miscellaneous changes for a v3 Tor link protocol
Author: Nick Mathewson
Created: 19-Sep-2011
Status: Closed
Target: 0.2.3.x

Overview:

  When proposals 176 and 179 are implemented, Tor will have a new
  link protocol.  I propose two simple improvements for the v3 link
  protocol: a more partitioned set of which types indicate
  variable-length cells, and a better way to handle link padding if
  and when we come up with a decent scheme for it.

Motivation:

  We're getting a new link protocol in 0.2.3.x, thanks (again) to
  TLS fingerprinting concerns.  When we do, it'd be nice to take
  care of some small issues that require a link protocol version
  increment.

  First, our system for introducing new variable-length cell types
  has required a protocol increment for each one.  Unlike
  fixed-length (512 byte) cells, we can't add new variable-length
  cells in the existing link protocols and just let older clients
  ignore them, because unless the recipient knows which cells are
  variable-length, it will treat them as 512-byte cells and discard
  too much of the stream or too little.  In the past, it's been
  useful to be able to introduce new cell types without having to
  increment the link protocol version.

  Second, once we have our new TLS handshake in place, we will want
  a good way to address the remaining fingerprinting opportunities.
  Some of those will likely involve traffic volume.  We can't fix
  that easily with our existing PADDING cell type, since PADDING
  cells are fixed-length, and wouldn't be so easy to use to break up
  our TLS record sizes.

Design: Indicating variable-length cells.

  Beginning with the v3 link protocol, we specify that all cell
  types in the range 128..255 indicate variable-length cells.
  Cell types in the range 0..127 are still used for 512-byte
  cells, except that the VERSIONS cell type (7) also indicates a
  variable-length cell (for backward compatibility).

  As before, all Tor instances must ignore cells with types that
  they don't recognize.

Design: Variable-length padding.

  We add a new variable-length cell type, "VPADDING", to be used for
  padding.  All Tor instances may send a VPADDING cell at any point that
  a VERSIONS cell is not required; a VPADDING cell's body may be any
  length; the body of a VPADDING cell MAY have any content.  Upon
  receiving a VPADDING cell, the recipient should drop it, as with a
  PADDING cell.

  (This does not give a way to send fewer than 5 bytes of padding.
  We could add this in the future, in a new link protocol.)

  Implementations SHOULD fill the content of all padding cells
  randomly.

A note on padding:

  We do not specify any situation in which a node ought to generate
  a VPADDING cell; that's left for future work.  Implementors should
  be aware that many schemes have been proposed for link padding
  that do not in fact work as well as one would expect.  We
  recommend that no mainstream implementation should produce padding
  in an attempt to resist traffic analysis, without real research
  showing that it helps.

Interaction with proposal 176:

  Proposal 176 says that during the v3 handshake, no cells other
  than VERSIONS, AUTHENTICATE, AUTH_CHALLENGE, CERT, and NETINFO are
  allowed, and those are only allowed in their standard order.  If
  this proposal is accepted, then VPADDING cells should also be
  allowed in the handshake at any point after the VERSIONS cell.
  They should be included when computing the "SLOG" and "CLOG"
  handshake-digest fields of the AUTHENTICATE cell.

Notes on future-proofing:

  It may be in the future we need a new cell format that is neither the
  original 512-byte format nor the variable-length format.  If we
  do, we can just increment the link protocol version number again.

  Right now we have 10 cell types; with this proposal and proposal
  176, we will have 14.  It's unlikely that we'll run out any time
  soon, but if we start to approach the number 64 with fixed-length
  cell types or 196 with var-length cell types, we should consider
  tweaking the link protocol to have a variable-length cell type
  encoding.

